In this experiment,, a physicist sits in front of a gun which is triggered or not triggered by radioactive decay. With each run of the experiment there is a 50-50 chance that the gun will be triggered and the physicist will die. If the Copenhagen interpretation is correct, then the gun will eventually be triggered and the physicist will die. If the many-worlds interpretation is correct then at each run of the experiment the physicist will be split into a world in which he lives and one in which he dies. In the worlds where the physicist dies, he will cease to exist. However, from the point of view of the physicist, the experiment will continue running without his ceasing to exist, because at each branch, he will only be able to observe the result in the world in which he survives, and if many-worlds is correct, the physicist will notice that he never seems to die.
Unfortunately, the physicist will be unable to report the results because, from the viewpoint of an outside observer, the probabilities will be the same whether many worlds or Copenhagen is correct.
A variation of this thought experiment suggests a controversial outcome known as quantum immortality, which is the argument that if the many-worlds interpretation of quantum mechanics is correct then a conscious observer never can cease to exist.